Tungsten carbide is a ceramic material having good chemical stability and corrosion resistance. It has been said to be the strongest of all structural materials, having high hardness, wear resistance and temperature resistance. Tungsten carbide powders are used to form cemented carbides, dies and cutting tools, wear resistant parts, cermets and electrical resistors, and are used as abrasives in liquids. Cemented tungsten carbides are mixtures of about 80 to 95 percent by weight tungsten carbide and about 5 to 20 percent by weight cobalt or other ductile metal as a binder phase. Cemented tungsten carbides are useful for forming tools and as abrasives for machining and grinding metals, rock, molded products, porcelain and glass. Cemented tungsten carbides are also useful in gages, blast nozzles, knives and drill bits.
Articles can be manufactured from tungsten carbide powder in a variety of ways. The most common ways of forming articles from tungsten carbide are hot pressing, cold pressing followed by sintering, and slip-casting (paste molding) followed by sintering. The method disclosed in U.S. Pat. No. 5,041,261 (Buljan et al.) is a recent example involving presintering followed by hot isostatic pressing.
The properties ultimately possessed by such articles are dependent to a great extent upon the grain size of the tungsten carbide employed and the conditions of sintering. Certain tungsten carbides of fine grain or particle sizes (known as micrograin tungsten carbides) from 0.05 to 0.2 micrometers in diameter are especially useful for certain purposes, such as end milling and circuit board drilling applications.
Particularly with micrograin materials, it is preferred to employ particles having a controlled morphology, a narrow size distribution, a well-defined stoichiometry and relatively high purity. While there are a variety of processes known for preparing tungsten carbide powders, many of them achieve particle sizes well above micrograin size. Micrograin tungsten carbide is not readily prepared from larger tungsten carbide particles, because the same hardness for which tungsten carbide is valued also impairs or prevents the mechanical reduction of large tungsten carbide particles to a smaller size by conventional methods such as grinding or milling. For example, it has been known to form small particles of other metal carbides by the single-step carbothermal reduction of a finely divided mixture of the metal oxide with carbon at a high temperature under a protective atmosphere, in which the resulting lumps of metal carbide are broken up by jaw crusher and then finely milled into powders of appropriate size. The hardness of tungsten carbide makes such a method unsuitable for the production of micrograin tungsten carbide. Accordingly, it would be very desirable to directly manufacture tungsten carbide particles having a small initial size, rather than to create an intermediate tungsten carbide which must then be reduced in size.
Is is known that some metal carbides cart be produced by the carbothermal reduction of a metal oxide with solid carbon under a reducing atmosphere, for example, under a hydrogen atmosphere. However, the direct carbothermal reduction of tungsten trioxide (or another solid, tungsten-containing material) with carbon in a molecular hydrogen atmosphere has not conventionally been viewed as possible. For example, it is noted in Funtai Oyobi Funmatsuyakin, v. 22(1), pp. 12-16 (1975) (Chemical Abstracts 88(24):175199r) that the reduction of tungsten trioxide by hydrogen occurs preferentially over the reduction of tungsten trioxide by carbon when a mixture of tungsten trioxide and carbon is heated in an atmosphere of molecular hydrogen. The final reduction step, the reduction of tungsten monoxide to tungsten metal by molecular hydrogen, produces water which reacts with the carbon that is present. The quantity of water affects the carbon content of the tungsten carbide ultimately produced. It is the belief of the present Applicants that the partial pressure of water is an important factor in the material transport for growth of tungsten crystal particles which are subsequently converted to tungsten carbide, and that the most significant reaction is that of water vapor with tungsten trioxide directly, yielding relatively volatile ortho- tungstic acid.
A variety of other processes have also been employed to make fine tungsten carbide powders. While they have each enjoyed relative success, they have also individually been subject to a variety of drawbacks. In general, the prior processes have been relatively expensive in requiring substantial capital investment in gas composition control systems and particle collection systems, while yielding products having large variations in particle size and relatively low yields. While these drawbacks are particularly evident with gas-phase methods (entailing the reaction of at least two gaseous reactants heated in a furnace, or heated by RF induction or plasma) they are also present in other methods.
U.S. Pat. No. 4,172,808 (Bohm et al.) is directed to a method for forming tungsten carbide by directing a mixture of carbon monoxide and carbon dioxide over tungsten oxide, while heating it in a heated reactor at a heating rate and gas flow rate such that the reduction of the tungsten oxide occurs more slowly than the diffusion of the carbon into the tungsten. Funtai Oyobi Funmatsuyakin, v. 26(2), pp. 72-77 (1979) (Chemical Abstracts 91(10):77333x) discloses a process for the direct production of tungsten carbide from tungsten oxide, entailing heating a granulated mixture of tungsten oxide and carbon powder in a rotary carburization furnace having graphite furnace elements. The powder was continuously fed into the furnace at a temperature of 1150.degree. to 1650.degree. C. It was asserted that the carbon content of the carburized product could be controlled by controlling the ratio of the partial pressure of the carbon dioxide to the partial pressure of the carbon monoxide formed during the reaction. Each of these references possesses the disadvantage that the partial pressures of carbon monoxide and/or carbon dioxide gas must be controlled throughout them.
U.S. Pat. No. 4,008,090 (Miyake et al.) discloses a two-step process for the production of tungsten (or a mixed metal) carbide, in which reduction of tungsten oxide to tungsten takes place in the presence of carbon black in an inert gas at 1000.degree. to 1600.degree. C., while carburization of the tungsten thereafter takes place in a hydrogen atmosphere at 1400.degree. to 2000.degree. C. The first step yields a mixed carbide of tungsten, ditungsten carbide and tungsten carbide. (The reference discloses a comparative example in which tungsten oxide and carbon black are carbonized in a hydrogen atmosphere at 1400.degree., 1600.degree. or 1800.degree. C. but which yields an unsatisfactory product.) This process is disadvantageous in requiring the isolation of an intermediate and requiring two different heating atmospheres at high temperatures.
U.S. Pat. No. 4,115,526 (Auborn et al.) is directed to a method for producing fine particle size tungsten or tungsten carbide which entails a first step of reducing tungsten oxide in hydrogen under conditions which avoid the formation of tungsten oxide whiskers, the resulting powder then being subject to carburization in a methane atmosphere. This process is disadvantageous in requiring two different atmospheres.
U.S. Pat. No. 4,460,697 (Hara et al.) discloses a method for synthesizing tungsten carbide from tungsten oxide by heating tungsten oxide to greater than 800.degree. C. in the presence of reducing and carbonizing gases, either sequentially or at the same time. When both reducing and carbonizing gases are employed at the same time, the gases are placed in a plasma condition. Fine tungsten carbide particle sizes on the order of 0.1 micrometers are obtained from the rapid cooling (10,000.degree. C. per second or faster) of the tungsten oxide from a plasma state prior to carbonizing. The disclosed process is disadvantageous in requiring plasma-condition treatment of the reactant material at some time during the process. The disclosed process also fails to obtain micrograin tungsten carbide in particularly useful sizes, below 0.3 micrometers, and especially below 0.1 micrometers. Moreover, like other plasma methods, the reference requires close control of the large thermal and reactant concentration gradients within the plasma reaction zone. Plasma methods entail the use of a large volume of gas through the reaction zone, and require large collection systems such as filters or electrostatic precipitators. Low yields and large variations in particle size are encountered as well, due to the temperature gradients.
U.S. Pat. No. 4,851,206, to Boudart, et al. discloses a method for making high specific surface area carbides by thermal reduction of oxides in the presence of a source of carbon with relatively slow progressive temperature increases prior to completion of the reaction, followed by quenching. Tungsten carbide is listed as one of the carbides that may be formed by Boudart, et al's method, although no process temperatures are recited in the patent for making tungsten carbide.
Boudart, et al. compare the results of their method of carburizing an oxide to form a carbide to results of carburizing a different reactant, i.e., the elemental metal, to form a carbide. Specifically, Boudart, et al. teach that the carbide product formed by their method has a substantially higher specific surface area than is obtained when the elemental metal is contacted with a carbon source at a fixed temperature and the reaction allowed to go to completion.
The carbide product formed in Boudart, et al. is said to have a specific surface area of at least 40 m.sup.2 /g and generally have a particle size in the range of about 1 to 15 nanometers, which is useful when used as a catalyst. However, tungsten carbide having such a high surface area and small particle size is not desirable in many processes or applications.
Firstly, tungsten carbide having a particle size of 1 to 15 nm is so highly reactive that it is pyrophoric. Being pyrophoric renders the material hazardous, in that it must be guarded against spontaneous burning or exploding.
Secondly, it is a fact that the oxide content of tungsten carbide is proportional to the particles' surface area. Tungsten carbide prepared by the method of Boudart, et al., that is, having a specific surface areas of at least 40 m.sup.2 /g, is difficultly, if not impossibly, used for making wear-resistant articles, as the parts made by such powders would be too weak due to the high oxide content.
The tungsten oxide used in the Boudart, et al. process is said to generally have a specific surface area from 0.1 to 10 m.sup.2 /g. Tungsten oxide having this particle size is not the most widely available tungsten oxide.
It is therefore an object of the present invention to convert tungsten trioxide or another solid, non-elemental tungsten-containing material to monotungsten carbide directly with a substantially single reaction atmosphere of substantially consistent composition.
It is a further object of the present invention to produce micrograin monotungsten carbide of good purity and relatively uniform particle size without requiring an appreciable investment in atmospheric controls or particle collection systems.
It is yet another object of the present invention to provide a method of producing micrograin monotungsten carbide at a relatively low reaction temperature.
It is another object of the present invention to provide a relatively inexpensive method of producing monotungsten carbide which is simpler than prior methods, yet which obviates the need for isolation or cooling of an intermediate product; for the addition of carbon to an intermediate product; for control over the ratio of the partial pressures of carbon monoxide and/or carbon dioxide in the treatment atmosphere; and for changing the proportions of the atmosphere during treatment.
It is also an object of the present invention to form tungsten carbide having a particle size from about 50 to about 200 nanometers, which is excellent for manufacturing wear-resistant articles (e.g. cutting tools and end mills) and is non-pyrophoric and, therefore, safe to use.
It is also an object of the present invention to employ widely available tungsten oxide particles having a surface area of 0.01 to 0.09 m.sup.2 /g.
It is yet another object of the present invention to provide a method of making tungsten carbide capable of being used in a continuous mode, thereby making the manufacturing of tungsten carbide commercially viable.